In particle-beam lithography apparatus and methods, a desired pattern is written or projection-exposed on a substrate such as a semiconductor wafer coated with a particle-beam-sensitive resist (hereinafter referred to simply as a "wafer"). Examples of particle beams include protons, ions, and electrons.
The most common particle beam used for such purposes is an electron beam. The electron beam is made incident on the resist in a controlled way. Whenever electrons enter the resist, they lose energy and experience trajectory changes via elastic and inelastic collisions known collectively as "electron scattering." Because scattered electrons can still cause exposure-related changes in the resist, electron scattering can reduce the resolution of pattern linewidths or other desired features to be defined by the electron beam on the wafer. This effect is known in the art as "proximity effect." The main type of scattering responsible for the proximity effect is back-scattering of electrons from the electron beam incident upon the electron-beam-sensitive substrate.
To compensate for the proximity effect, it is known in the art to make substantially equal, over the entire surface of the electron-beam-sensitive substrate, the exposure of the substrate caused by these backscattered electrons.
To such end, it is known in the art to employ a "compensation mask" particularly with projection exposure of a particle-beam-sensitive substrate using a particle beam. In such a scheme, the pattern to be transferred (as defined on a mask or "reticle") to the substrate is divided into many subfields. The compensation mask, which is separate from the reticle, is also divided into subfields (preferably in register with the subfields on the wafer). At each subfield on the compensation mask, an aperture through the thickness dimension of the mask is defined having an area that is equal to the area of the nominally unexposed regions of the corresponding subfield region on the substrate. With this method, by keeping the compensation mask and the substrate separated a prescribed distance from each other, uniform compensation exposure of each of the subfields is performed by blurring the energy beam (ultraviolet ray, electron beam, etc.) at the substrate by passage of the beam through the corresponding apertures in the compensation mask.
In the prior art described above, it is possible to favorably compensate for proximity effect whenever a positive resist is used. However, whenever a negative resist is used, it is not possible to achieve high precision of the pattern transferred to the wafer, despite attempts to compensate for proximity effects. More specifically, with a positive resist, if the compensating energy strikes a nominally unexposed region, there is only a small reduction in the resist-film thickness at that region, with no adverse effect on pattern precision. In contrast, with a negative resist, if the compensating energy strikes a nominally unexposed region, some residual resist--a negative resist defect--can remain after developing the resist, making high-precision pattern formation impossible.
Furthermore, with conventional proximity effect compensation methods, there is no particular mechanism for aligning the compensation mask and the substrate. Such alignment has been performed, for example, with a microscope or the like by viewing an open pattern at a prescribed area on the mask and the corresponding pattern on the substrate. But, as the degree of integration of transferred patterns has increased, it has become desirable to increase the precision of alignment of the compensation mask and the substrate, and perform compensation for the proximity effect in minute and close fashion in correspondence with the degree of refinement in the pattern at the various areas on the substrate.
Compensation exposure of the entire surface of the particle-beam-sensitive substrate has conventionally been conducted, for example, by scanning an energy beam of prescribed width across the compensation mask. Unfortunately, the precision by which such scans are placed adjacent each other is typically poor, causing problematic unevenness in compensation exposure at boundary regions between scans.